The invention relates to correcting non-uniformity in displays such as projection displays.
A liquid crystal display (LCD) includes an array of pixels that may be manipulated to present an image. A LCD panel typically includes two glass plates and a liquid crystal material between them. One application of LCDs is in projection displays, in which one or more LCDs may be used to modulate the intensity or polarization of light from a light source. In a color projection display, multiple LCDs may be used, one for red, one for blue, and one for green, for example. The images generated by each of the LCDs are aligned and combined through a lens assembly and projected onto an external screen.
Some LCD projection systems use reflective LCD panels, in which each LCD panel modulates incident light beams and reflects the modulated beams so that the modulated beams return along predetermined paths. In this manner, the modulated beams may be directed through a lens assembly to form images on a display screen that add together to form a color composite image.
Reflective LCD panels may contain certain non-uniformities that are caused by the semiconductor processes used to manufacture reflective substrates in the reflective LCD panel. For example, such non-uniformities may be caused by different transistor gains, different storage capacitor values, leakage currents, and etching variations on the pixel mirror surface. As a result, subtle variations in light output from the pixels of each reflective LCD panel may be introduced. In addition, the projection lens assembly may not be completely free of variation in its flatness of field, which may further add to non-uniformity of a displayed image.
One way to correct for these non-uniformities is to measure the light output of each pixel and compare it to the expected light output. It might be difficult to single out a particular pixel and may be easier to measure the entire light output of the display. However for a 1024.times.768 display there is one pixel being measured and 786431 which are not, but still influence the measured result. Measuring a pixel to one part in 256 when each pixel is one part in 786432 makes high demands on the measurement accuracy.
Such non-uniformities between pixels of a displayed image may become obvious to a viewer if the viewer watches the image on the screen for some amount of time, such as more than a few seconds. Making the needed calibration measurements is difficult and requires very high accuracy circuits.
Thus, a need exists for a technique and apparatus to make the measurement with sufficient accuracy so that the non-uniformity may be calibrated out of the display system.